1. Field of the Invention
This invention relates to spark ignition systems for gas turbine and diesel engines that operate on diesel, natural gas or alternative fuels and require at least an initial ignition source.
2. Description of the Prior Art
Current gas turbine engines for power production such as those used for hybrid electric vehicles and power generation require very high energy spark ignition systems due to use of low volatility fuels that are difficult to ignite. Typical high energy ignition systems are those used in the avionic industry for auxiliary power units (APUs). Some of these systems have severe emission control requirements that can be met only by providing very high energy ignition sources in order to start the engine before too much unburned fuel is released through the exhaust system. Diesel engines require glo-plugs to initiate combustion. In this case the glo-plug tip is heated to temperatures of  greater than 2000xc2x0 F. which typically takes large amounts of current (xcx9c8 amps per plug) and lengthy warm up times.
To achieve the spark ignition performance needed for ignition and, at the same time, reduce the incidence of spark plug soot fouling, the spark ignition transformer core material must possess certain properties. Such core material must have moderately high magnetic permeability, must not magnetically saturate during operation, and must have low magnetic losses. The combination of these required properties severely curtails the availability of suitable core materials. Possible candidates for the core material include silicon steel, ferrite, and iron-based amorphous metal. Conventional silicon steel routinely used in utility transformer cores is inexpensive, but its magnetic losses are too high. Thinner gauge silicon steel with lower magnetic losses is too costly. Ferrites are inexpensive, but their saturation inductions are normally less than 0.5 T and Curie temperatures at which the core""s magnetic induction becomes close to zero are near 200xc2x0 C. This temperature is too low because a spark ignition transformer""s upper operating temperature is typically about 180xc2x0 C. Conventional iron-based amorphous metal has low magnetic loss and high saturation induction exceeding 1.5 T, however it shows relatively high permeability, limiting its energy storage capability.
Conventional avionic ignition systems can deposit very high energies (500 millijoules) into the spark, but typically operate at 10 Hz or less due to power consumption issues and also require DCxe2x80x94DC converters. They also have high rates of ignitor erosion, limiting the total duration of operation between ignitor changes and precluding their being operated continuously.
The present invention provides an ignition system containing a magnetic core-coil assembly and associated driver electronics. The system is capable of high pulse rate operation because of its rapid charge time (for example, xcx9c100 microseconds using a 12 volt source), rapid voltage rise (for example, 200-500 nanoseconds), and rapid discharge time (for example, xcx9c150 microseconds). It has low output impedance (30-100 ohms), produces high ( greater than 25 kV) open circuit voltages, and delivers high peak current through the spark (0.4-1.5 ampere) and high spark energy, typically 6-12 millijoules per pulse. Operation from a 12 volt battery source is readily accomplished using simple driver electronics at rates ranging from single shot to about 4 kHz, which are considerably greater than the current ignition systems can offer. The core-coil assembly may actually be operated using any voltage  greater than 5 volts to supply the driver electronics input voltage. The upper voltage supply limit is dependent on the voltage rating of the components used within the driver electronics, so the present system may be operated with conventional 12 V power or with readily available components at higher supply voltages including the 40-50 Volt system now being contemplated within the automotive industry. The charging time of the core-coil assembly is related to the supply voltage of the driver electronics. The higher the supply voltage, the faster the current will increase through the primary winding of the core-coil. This is due to loss reduction in the components that comprise the driver electronics and the ability to source more current. At lower voltages, the voltage drop across the switching element of the driver electronics (typically an IGBT) will limit the available voltage drop across the core-coil. This has the effect of increasing the charge time until a pre-determined current is flowing through the core-coil primary. This type of electronic system (electronic driver plus core-coil) output delivered through a surface gap plug (typical of avionic spark ignition systems) or a conventional J gap spark plug or derivatives results in a high power ignition source with localized heating capability. A xe2x80x9cspark plugxe2x80x9d or alternative term xe2x80x9cignitorxe2x80x9d refers to a device that requires high voltage to create a spark across a gap. That gap can be a ceramic which is typical of a surface gap ignitor, or it can be an air gap, which is typical of a xe2x80x9cJxe2x80x9d gap spark plug. A xe2x80x9cJxe2x80x9d gap derivative refers to any other type of spark plug where an arc must be created over a distance similar to the distance between electrodes of a conventional xe2x80x9cJxe2x80x9d gap spark plug.
The magnetic core-coil assembly and ignition system of the invention may be operated at much higher pulse rates than prior art systems. The high pulse rates have a number of advantages applicable both to turbine and to diesel engines. Avionic systems are capable of high energy per spark but typically achieve only a 10 Hz rate. In the case of turbine engines fuel is burned substantially continuously. During engine start-up an ignition source must be provided. This source may advantageously employ an ignition system with a very high pulse rate, such as the 4 kHz or more that the present system can provide. The system is generally operated asynchronously, that is, spark activation is not synchronized to the position of other moving parts in the engine. After the engine is running continuously, the ignition system may be turned off, since the fuel burning is normally self-sustaining. However, in applications such as aerospace, safety considerations may dictate that the ignition system be activated at least periodically to insure the engine continues to run despite adverse conditions. For example, the intake of moisture into an aircraft turbine propulsion engine can cause a flameout, that is the quenching of the self-sustaining reaction, necessitating an engine re-start. For example, a gas turbine engine may flame out when an aircraft flies through rain. To avoid this, the ignition system may periodically be activated during known adverse conditions. However, the high Coulombic transfer of energy in a conventional system results in very rapid erosion of spark ignitors, thereby limiting the duty cycle and extent of the periodic activation of the system. In contrast, the present system experiences substantially slower rates of ignitor degradation, so the extra ignition can be used much more liberally, enhancing flight safety without the risk of ignitor failure.
The high pulse rate arc obtainable with the present system can also act as a localized heating source that can be activated essentially instantaneously, thus representing a cost effective replacement for glo-plugs in some applications such as diesel engines. The high pulse ( greater than 300 pulses per second) rate arc can create a greater heating of the fuel droplets or gas since the amount of total energy in the multiple arcs can exceed that of a conventional ignition system which is limited to approximately 110 pulses per second. In a diesel automotive or truck vehicle application, the engine may thus be started essentially on demand without the waiting time for a glo-plug to heat. In addition, a smaller battery may be used, since the total energy required for glo-plug heat up is much greater than the present system uses in start-up.
Generally stated, the magnetic core-coil comprises a magnetic core consisting of a ferromagnetic amorphous metal alloy. The core-coil assembly has a single primary coil for low voltage excitation and a secondary coil for a high voltage output. A number of core forms are possible, including both a single core with a single primary and a single secondary and a multiple core form such as the core included in the magnetic core-coil assembly described in detail in U.S. Pat. No. 5,844,462 which is assigned to the assignee of the present application and is hereby incorporated by reference into this disclosure. The latter core-coil version is known to those in the art as a pencil coil and will be referred to as such in this disclosure. This assembly has a secondary coil comprising a plurality of core sub-assemblies that are simultaneously energized via the common primary coil for a time during which current flows in the primary, storing energy in a magnetic field within the core material. The core sub-assemblies are adapted, when energized by the driver electronics, to produce secondary voltages. That is to say, during the period that the sub-assemblies are energized by the driver electronics, the primary current is rapidly interrupted, causing the magnetic field within the cores to collapse. Secondary voltages are thereby induced across the each of the secondary windings. These secondary voltages are additive in the pencil coil design, and the voltage is fed to the spark plug via the secondary connection to the spark plug or ignitor.
The single core-coil embodiment has a single primary and a single secondary but operates similarly. Energy is stored in the magnetic core as a result of current flowing through the primary. When the primary current flow is rapidly interrupted by the driver electronics, the magnetic field within the core collapses. A voltage is thereby induced and appears across the single secondary, which is connected to the spark plug or ignitor.
Compared to cores made with prior art materials, cores of the invention made with ferromagnetic amorphous metal alloy require fewer primary and secondary windings due to the magnetic permeability of the core material and exhibit lower magnetic losses. As thus constructed, the core-coil assembly has the capability of generating a high voltage in the secondary coil within a short period of time following excitation thereof.
More specifically, the core consists of an amorphous ferromagnetic material which exhibits high saturation magnetization, low core loss and a permeability ranging from about 100 to 500. The lower the core""s permeability, the higher the energy that can be stored in the magnetic field and made available to be converted into spark energy, but also the higher the required magnetomotive force (amp-turns) and, hence, current. The magnetic properties recited are especially suited for rapid firing of the plug. Misfires due to soot fouling are minimized. Moreover, energy transfer from coil to plug is carried out in a highly efficient manner, with the result that very little energy remains within the core after discharge. The low secondary resistance of the toroidal design ( less than 100 ohms) allows the bulk of the energy to be dissipated in the spark and not in the secondary wire. In a pencil coil design, a multiple toroid assembly is created that allows energy storage in the sub-assemblies via a common primary governed by the inductance of the sub-assembly and its magnetic properties. A rapidly rising secondary voltage is induced when the primary current is rapidly decreased. The individual secondary voltages across the sub-assembly toroids rapidly increase and add sub-assembly to sub-assembly, based on the total magnetic flux change of the system. This provides for a versatile arrangement in which several sub-assembly units are combined. The sub-assembly units are wound using existing toroidal coil winding techniques to produce a single assembly with superior performance in cases where physical dimensions are critical.
Another embodiment uses a single larger toroidally wound core-coil that produces output characteristics similar to those of the pencil coil (multiple stack arrangement of smaller core-coil assemblies) described above. The unit operates in the manner described above. Use of a single core is attractive because of its simpler manufacture and the typically lower resistance of the windings for a given core cross-sectional area.
The driver electronics comprise a power source (typically a battery), a low Equivalent Series Resistance (ESR) capacitor to supply high peak current, a switch such as an Integrated gate bipolar transistor (IGBT) which can be turned on (shorted condition) to allow current to flow through the coil primary establishing the magnetomotive force and then subsequently turned off (open condition) which rapidly decreases the current flow through the primary of the coil causing the magnetic field to collapse in the core inducing voltage onto the secondary winding producing an output. A timing means may be required to turn the switch on and off at the appropriate times.